Homes are often built with crawl spaces underneath substantially all or a significant portion of the structure. The crawl space is typically left in a natural state with open soil left in the condition that existed prior to and after the building was completed. The foundation of a home is usually concrete and cinder blocks that support lumber flooring, wall supports and the roof. Rain and snow permeate the soil, so moisture easily develops in the space between the ground and the flooring which is destructive to the materials used in building the foundation and unhealthy for the home occupant.
Soil in most parts of the United States has a high moisture content. This moisture will readily work its way from the soil up into the crawl space, eventually into the structure of the house, and finally into the living space. Crawl spaces in southeastern United States homes where humidity levels are typically high and the soil has a relatively high moisture content have been studied extensively with respect to moisture issues. Though the crawl space is a convenient place for locating plumbing, heating, air conditioning systems, and ductwork, moisture will gather in this area and will move into the living area of the house.
Numerous types of mold will thrive in a damp and moist environment, reproducing millions of spores that will ultimately reach the home living space by airflow that naturally carries them upward from a damp crawlspace. Heat causes an air mass to rise. Air located in the crawl space will become warm by being located near sources of heat in a residence. Once the crawl space air masses are heated, they move into living areas. Since relative humidity levels in most crawl spaces are in the range of from 50-90%, mold has a receptive environment in which to easily develop and spread. Mold can grow on dirt, construction debris, house framing, and under carpeting. Mold will feed on organic materials and then destroy those materials as it continues to feed and grow.
Some techniques for reducing moisture build up in a crawl space involve ventilating the area. Ventilation is accomplished passively by providing openings in the foundation to allow airflow from the crawlspace to the outside air and mechanically by installing air circulating fans. Heating and cooling costs can be significantly higher for homes that have a crawl space vented by either a passive or a mechanical air flow system.
Passively vented crawl spaces, the most common form of crawl space treatment, are the most prone to accumulate moisture and to use the most energy.
As a means to promote energy efficiency, the US Department of Energy recommends that if the crawlspace is to be unvented, all holes in the foundation where outside air can enter should be sealed. It is recommended that a polyethylene vapor retention member, or equivalent material, should be installed over a dirt floor. The polyethylene barrier can be covered with a layer of sand or concrete to protect it from damage, but gravel, or anything that could pierce the barrier, should not be employed. Another recommendation is to run the plastic vapor barrier up the walls forming the crawl space, and this plastic barrier should be attached to the first piece of wood (the sill plate or mudsill). Rigid insulation foam board should be installed against the foundation from the subfloor to the plastic (or concert slab) on the floor of the crawlspace. It is suggested that this be done around the perimeter of the foundation. An alternate to the use of foam board is to drape fiberglass roll insulation down the foundation walls with edges butted tightly together. Roll fiberglass insulation is commonly used between the floor joists. This is an acceptable alternative to foam board insulation as long as the crawl space remains dry. However, some state building codes require inspection gaps between the sill plate and the start of the encapsulation material. This means that the vapor barrier material cannot be attached directly to the sill plate.
Some systems that offer a dry crawl space utilize material such as 0.006-0.012 inch thickness polyethylene film to provide a vapor barrier. Such systems eliminate crawl space in and out airflow, completely encapsulate the ground and the walls of the crawl space and prohibit any passive airflow in it. Using this encapsulation system, energy expenses can be reduced by more than 15% each year. These systems provide benefits in moisture management, pest control, radon reduction, and thermal insulation. The most important function is providing an airflow barrier, and the most significant shortcoming is not providing enough thermal insulation.
One system providing complete crawlspace encapsulation uses a 0.020 inch high density polyethylene and low density material formed of seven blended layers. This product is very similar to the material used as swimming pool liners. A product sold as the CLEAN SPACE CRAWL SPACE ENCAPSULATION SYSTEM has been utilized and successfully eliminated moisture that leads to the growth of unhealthy mold from entering living areas. CLEAN SPACE and CRAWL SPACE ENCAPSULATION SYSTEM are registered trademarks of Basement Systems, Inc. However, the material is relatively heavy (a roll typically weighing 110 pounds) and is difficult to handle in the confines of a crawlspace during installation.